GOLD DEPOSITING MECHANISMS IN HIGH-GRADE ORE SHOOTS OF THE PAMOUR MINE, TIMMINS, ONTARIO, CANADA

High-grade ore shoots in mesothermal gold deposits of the Pamour Mine occur in two distinct settings: shallow-dipping (<45°) and steep-dipping (>45°) veins. High-grade ore shoots in the economically important steep-dipping veins were formed by phase separation as indicated by the range in composition of individual fluid inclusions extracted from gold, by inclusion fluids in both gold and quartz that have CO₂:CH₄ ratios that increase with H₂O contents, and by the concentration of gold on the footwall of individual sheets. The preferential concentration of gold in the thinnest parts of individual sheets (by a factor of 5 to 1,000 compared to wider sections of the same sheet) suggest that fluid turbulence at constrictions in hydrothermal fractures induced phase separation and gold deposition. Pressures estimates made using the composition of gold-hosted and quartz-hosted inclusion fluids are £0.4 kbars and ~1/3 that determined for shallow-dipping veins, suggesting that steep-dipping veins formed at pressures close to hydrostatic. Deposition of gold by fluid turbulence is also an attractive scenario because it could explain the great vertical continuity characteristic of most mesothermal gold deposits.

Gold concentrated near fractures in the hanging wall and on surfaces of wallrock fragments in massive or brecciated shallow-dipping veins suggest that fluids reacting with the wallrocks formed high-grade ore shoots. This possibility is supported by a decrease in CO₂:CH₄ ratios with increasing H₂O contents of individual gold-hosted fluid inclusions and carbonate alteration of the wallrocks. Pressure estimates of 1.2 kbars, made using the composition of water-rich fluid inclusions, suggest that the gold was deposited when fluid pressures were close to lithostatic. Shallow-dipping stringer veins also appear to have formed by the interaction of hydrothermal fluids with the wallrocks as indicated by a 1‰ decrease in d¹⁸O_quartz values between foot and hanging walls of a quartz stringer, and strong carbonate alteration of breccia fragments and hanging wall rocks.

The complex interplay of two major ore depositing processes, phase separation and wallrock reaction, during the formation of high-grade ore shoots at the Pamour Mine could have more widespread application to the development of mesothermal gold deposits worldwide.